This invention relates to bipolar batteries of the zinc-bromine type and, more particularly, to bipolar batteries of the zinc-bromine type having shunt current protection systems.
Bipolar electrochemical flow battery systems are well known. See, for example, U.S. Ser. No. 189,363 entitled "Terminal Electrode" filed May 2, 1988 by J. Zagrodnik and G. Bowen, the disclosure of which is hereby incorporated by reference.
Numerous techniques have been developed to reduce or minimize the undesirable side effects caused by the occurrence of bypass or the so-called "shunt currents" occuring in bipolar batteries wherein aqueous anolyte and catholyte are fed by manifolds to the various respective cells in a bipolar battery. Shunt currents occur as a result of the current seeking other conductive passages such as the manifolds used to feed the aqueous anolyte and catholyte in addition to the desired passage through the series connected cells. An undesirable side effect is the poor distribution of zinc causing poor battery performance or shortened battery life.
A particularly interesting technique for reducing shunt currents is embodied in the description of U.S. Pat. No. 4,197,169 issued to Zahn et al. on Apr. 8, 1980 in which a protective electric current is induced in the manifolds which carry the liquid electrolyte to the bipolar cells. In U.S. Pat. No. 4,279,732 issued to Bellows et al. on July 21, 1981, the electrodes providing the protective current, i.e., "protective electrodes", are made annular in shape about the manifolds providing a substantially uniform current density profile through the electrolyte manifold but does not impede the electrolyte flow itself.
U.S. Pat. No. 4,285,794 issued to Bellows et al. on Aug. 25, 1981 is directed toward the structure of the protective electrodes in a system for reducing shunt currents in zinc-bromine type bipolar batteries. Specifically, the annular negative electrodes have an outer sleeve and an inner porous liner. The outer sleeve is fed a bromine-rich electrolyte for the purpose of supplying the electrolyte flowing through the electrode with bromine but the inner porous sleeve does not permit the bromine-rich electrolyte itself to mix with the electrolyte. The liner, however, does permit the passage of ionic currents at low resistance.
U.S. Pat. No. 4,277,317 issued to Grimes et al. on July 7, 1981 describes the reduction of shunt currents in a bipolar battery by interconnecting the electrolyte channels (which communicate with the manifolds) that provide electrolyte to the cells with connecting tunnels. A protective current is formed along the tunnels and is supplied by an external source or the terminal cells of the battery itself. As described in U.S. Pat. No. 4,312,735, to Grimes et al., issued on June 26, 1982, power consumption of the tunnels is further reduced by tapering the tunnels so as to provide a smaller cross-section --and a higher electrical resistance--at the midpoint of the tunnels.
A typical construction of a bipolar battery of the zinc-bromine type is embodied in a unitary structure having a thermoplastic box-like frame in which an assembly of conductive substrates are positioned adjacent thin microporous separators having electrolyte channels embossed thereupon. The substrates alternate being coated with positive and negative materials. The entire assembly is tightly fitted within the external frame and sealed together. A plurality of manifold tunnels extending through the entire frame and assembly connect the aqueous catholyte and anolyte reservoirs to the proper catholyte and anolyte channels. Pumps supply the proper flow to the liquids. Each of the end blocks of the frame is provided with an external terminal lug which is electrically connected to the terminal electrodes within the frame. Additionally, a plurality of anodic and cathodic shunt tunnels interconnecting the respective channels extend from one end block to another. Protective negative and positive electrodes are positioned on the end blocks and are electrically connected to the shunt tunnels as described in the aforementioned U.S. Pat. No. 4,277,367 to Grimes, et. al. As stated in the aforementioned U.S. Pat. No. 4,312,335, the tunnels may be tapered to reduce power consumption.
Because of the build-up of zinc dendritic material and corrosive effects of the bromine, it is desirable that the protective electrodes and associated components be easily and quickly replaced at appropriate intervals. None of the references cited above, however, provide for or teach the removal and replacement of the protective electrodes in an economic and expedient manner.